The present invention relates to a computing system, a host computer, storage device used in the computing system and a volume switching method employed in the computing system.
To reduce the risk of data losses in a storage device due to accidents or failures, a technique called “host-free copy” is applied. This technique enables data of a storage device to be copied within the device itself or to another storage device without the mediation of a host computer. The data to be copied are handled in a unit of a storage area called a “logical volume” or “volume”. This logical volume is a logically separated area in the storage devices.
When a script or an application determines an area to which a file is allocated, a host computer needs a host identifier (host ID). This host ID is used to identify a logical volume uniquely by a host computer. A host ID differs from an intra-storage ID used to allow a storage device to identify a logical volume therein. In addition, the host ID and the intra-storage ID are related to each other in a pre-setting process called “storage device generation”.
The Host IDs typically correspond to intra-storage IDs in a one-to-one relation. However, a technique in which use of a single host ID enables the access to multiple logical volumes is disclosed in Japanese Unexamined Patent Application Publication No. 11-327986. In this technique, a host computer switches a relation between a logical volume and a host ID. Specifically, when an application issues an input/output (I/O) request, a host ID related to a certain logical volume makes it possible to access another logical volume to which another host ID is related.
It is sometimes difficult to make host IDs corresponding to intra-storage IDs in a one-to-one relation. This may occur when the number of allocatable host IDs is different from that of allocatable intra-storage IDs. The number of allocatable IDs is defined by the capacity of an address area, and this capacity is typically determined by a standard specification or something similar. In other words, if a host computer and a storage device are consistent to a certain standard specification, an address area of a predetermined capacity is used in accordance with the specification. This means that the number of their allocable IDs is limited by the specification.
As for a host computer, it uses a host ID in accordance with a standard specification in order to access various storage devices produced by individual venders. In contrast, a storage device uses an intra-storage ID in accordance with a specification defined arbitrarily by venders. This is because the intra-storage ID is used only for internal processing of the storage device.
Accordingly, a host ID in compliance with a standard specification confines an address area to a small capacity, and an intra-storage ID in accordance to an arbitrary specification enables an address area of a large capacity to be used. As a result, their capacities differ from each other. In other words, owing to the number of allocatable hosts ID, a host computer cannot access the whole storage areas of a storage device, thereby inhibiting the maximum use of the storage areas within the storage device.
Conventionally, host IDs for accessing a storage device are allocated to volumes of a storage device in a one-to-one relation. It is assumed that a host computer A uses a certain volume in order to execute a specific application and the copy of this volume is used by a host computer B for back-up or test use. In this case, upon a failure in the host computer A, the host computer B can inherit the operation of the host computer A by employing the technique disclosed in the above publication No. 11-327986. However, in order to achieve this operation, the host computer B needs to allocate a host ID beforehand not only to the target (copied) volume but also the source volume. In addition, when a target volume is already present, it is necessary to allocate a host ID to the target volume. This is because the host computer is necessary to access the target volume promptly, if a failure occurs in the source volume or if consistency is lacking between the source and target volumes.